Method of reducing wire bond profile height in integrated circuits mounted to circuit boards

ABSTRACT

A method of profiling a wire bond between a contact pad on a die, and a conductor on a supporting structure, by electrically connecting the contact pad on the die to the conductor on the supporting structure with a wire bond, the wire bond extending in an arc from the contact pad to the conductor, pushing on the wire bond to collapse the arc and plastically deform the wire bond, and then releasing the wire bonds such that the plastic deformation maintains the wire bond in a flatter profile shape. The strength of the wire bond is known to be relatively small; of the order of 3 to 5 grams force. However, the Applicant&#39;s work has found that the wire bond structure is robust enough to withstand a certain degree of work hardening from plastic deformation. The arc of the wire bond can be deformed into a flatter profile without compromising the electrical connection with the PCB.

FIELD OF THE INVENTION

The invention relates to the field of integrated circuit packaging. Inparticular, the encapsulation of the wire bonds between a circuit boardand the contact pads on the integrated circuit die.

CO-PENDING APPLICATIONS

The following applications have been filed by the Applicantsimultaneously with the present application: MPN007US MPN009US MPN010USMPN011USThe disclosures of these co-pending applications are incorporated hereinby reference. The above applications have been identified by theirfiling docket number, which will be substituted with the correspondingapplication number, once assigned.

CROSS REFERENCES TO RELATED APPLICATIONS

Various methods, systems and apparatus relating to the present inventionare disclosed in the following US patents/patent applications filed bythe applicant or assignee of the present invention:

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The disclosures of these co-pending applications are incorporated hereinby reference.

BACKGROUND OF THE INVENTION

Integrated circuits fabricated on silicon wafer substrates areelectrically connected to printed circuit boards by wire bonds. The wirebonds are very thin wires—around 25 to 40 microns in diameter—extendingfrom contact pads along the side of the wafer substrate to contacts onthe printed circuit board (PCB). To protect and strengthen the wirebonds, they are sealed within a bead of epoxy called encapsulant. Thewires from the contact pads to the PCB are made longer than necessary toaccommodate changes in the gap between the PCB and the contact padsbecause of thermal expansion, flex in the components and so on. Theselonger than necessary wires naturally form an arc between the contactpads and the PCB. The top of the wire arc is often about 300 micronsabove the contact pads although some wire bonding may extend evenhigher. As the name suggests, the encapsulant needs to encapsulate thefull length of the wire so the encapsulant bead will extend 500 micronsto 600 microns proud of the contact pads.

The integrated circuit fabricated on the silicon wafer is often referredto as a ‘die’. For the purposes of this specification, the term die willbe used as a reference to an integrated circuit fabricated on a wafersubstrate using lithographic the well known etching and depositiontechniques commonly used in semiconductor fabrication. If the die ispurely an electronic microprocessor, there is little need to keep closecontrol of the encapsulant bead dimensions. However, if the die is amicro-electro mechanical systems (MEMS) device with an active uppersurface, it may be necessary or desirable to bring the active surface ofthe die onto close proximity with another surface. One such situationapplies to inkjet printheads. The proximity of the print media to thenozzle array influences the print quality. Similarly, if a cleaningsurface is wiped across the nozzles, the bead of encapsulant can hamperthe wiping contact.

Another problems arises because of sides of the encapsulant bead are notstraight. One commonly used technique for depositing the encapsulantinvolves extruding it from a needle directly onto the line of wirebonds. The encapsulant volume and placement on the die is not veryaccurate. Variations in the pressure from the pump or slightnon-uniformities in the speed of the needle cause the side of the beadcontacting the active surface to be reasonably crooked. As the side ofthe bead is not straight, it has to be generously spaced from any activeparts on the active surface to comfortably accommodate theperturbations. Spacing the electrical contacts away from the activeportions (say for example, inkjet nozzles) of the active surface uses upvaluable wafer real estate and reduces the number of dies that can befabricated from a wafer disc.

In light of the widespread use of inkjet printheads, the invention willbe described with specific reference to its application in this field.However, the ordinary will appreciate that this is purely illustrativeand the invention is equally applicable to other integrated circuitswire bonded to a PCB or other support structure.

SUMMARY OF THE INVENTION

According to a first aspect, the present invention provides amicroprocessor device comprising:

a support structure having a chip mounting area and a conductor mountingarea;

a die supported on the chip mounting area, the die having a back surfacein contact with the chip mounting area and an active surface opposingthe back surface, the active surface having electrical contact pads;

a plurality of electrical conductors at least partially supported on theconductor mounting area; and,

a series of wire bonds extending from the electrical contact pads to theplurality of electrical conductors supported on the conductor mountingarea; wherein,

the chip mounting area is raised relative to the conductor mountingarea.

By raising the chip mounting area relative to the rest of the PCB, or atleast the conductors connected to the PCB end of the wire bonds, the topof the arc formed by the layer is much closer to the active surface ofthe die. This, in turn, allows the bead of encapsulant to have a lowerprofile relative to the active surface. With a lower encapsulant bead,the active surface can be brought into closer proximity with anothersurface without making contact. For example, the nozzle array on aprinthead IC can be 300 microns to 400 microns from the paper path.

Preferably, the chip mounting area is raised more than 100 micronsrelative to the conductor mounting area. Preferably, the supportstructure has a step between the chip mounting area and the conductormounting area.

Preferably, the plurality of conductors are incorporated into a flexibleprinted circuit board (flex PCB) with a line of bond pads along an edgeclosest the die, the bond pads being more than 2 mm from the contactspads on the die.

Preferably, the wire bonds are formed from wire with a diameter lessthan 40 microns and extend less than 100 microns above the activesurface of the die.

Preferably, the wire bonds are plastically deformed such that theyextend less than 50 microns above the active surface of the die.

Preferably, the active surface has functional elements spaced less than260 microns from the contacts pads of the die. In a particularlypreferred form, the die is an inkjet printhead IC and the functionalelements are nozzles through which ink is ejected. In some embodiments,the support structure is a liquid crystal polymer (LCP) molding.

Preferably, the wire bonds are covered in a bead of encapsulant, thebead of encapsulant extending less than 200 microns above the activesurface of the die.

Preferably, the wire bonds are covered in a bead of encapsulant, thebead of encapsulant having a profiled surface that is flat, parallel toand spaced less than 100 microns from the active surface.

Preferably, the wire bonds are covered in a bead of encapsulant, thebead of encapsulant having a profiled surface that is flat and inclinedrelative to the active surface.

Preferably, the wire bonds are covered in a bead of encapsulant, theencapsulant being an epoxy material that is thixotropic when uncured.

Preferably, the wire bonds are covered in a bead of encapsulant, theencapsulant being an epoxy material has a viscosity greater than 700 cpwhen uncured.

In a particular embodiment, the printhead IC is mounted in a printersuch that during use the nozzles are less than 100 microns from thepaper path.

According to a second aspect, the present invention provides a method ofprofiling a wire bond between a contact pad on a die, and a conductor ona supporting structure, the method comprising the steps of:

electrically connecting the contact pad on the die to the conductor onthe supporting structure with a wire bond, the wire bond extending in anarc from the contact pad to the conductor;

pushing on the wire bond to collapse the arc and plastically deform thewire bond; and,

releasing the wire bonds such that the plastic deformation maintains thewire bond in a flatter profile shape.

The strength of the wire bond is known to be relatively small; of theorder of 3 to 5 grams force. However, the Applicant's work has foundthat the wire bond structure is robust enough to withstand a certaindegree of work hardening from plastic deformation. The arc of the wirebond can be deformed into a flatter profile without compromising theelectrical connection with the PCB.

Preferably, the die has an active surface that has functional elements,the contacts pad being formed at one edge of the active surface, thewire bond has a diameter less than 40 microns and the arc extends morethan 100 microns above the active surface of the die.

Preferably, the wire bond is plastically deformed such that it extendsless than 50 microns above the active surface of the die.

Preferably, the wire bond is pushed by engagement with a blade having arounded edge section for contacting the wire bond.

Preferably, the method further comprises the steps of:

applying a bead of encapsulant over the wire bond; and,

moving a profiling surface over the active surface to flatten the beadof encapsulant.

Preferably, the bead of encapsulant having a profiled surface that isflat, parallel to and spaced less than 100 microns from the activesurface.

Optionally, the bead of encapsulant having a profiled surface that isflat and inclined relative to the active surface.

Preferably, the encapsulant being an epoxy material has a viscositygreater than 700 cp when uncured. In a particularly preferred form, theencapsulant being an epoxy material that is thixotropic when uncured.

Preferably, the method further comprises the steps of:

positioning the profiling surface adjacent and spaced from the activesurface to define a gap; and,

applying the bead of encapsulant onto the contact pads such that oneside of the bead contacts the profiling surface and a portion of thebead extends into the gap and onto the active surface.

Preferably, the active surface has functional elements spaced less than260 microns from the contacts pads of the die. In a particularlypreferred form, the die is an inkjet printhead IC and the functionalelements are nozzles through which ink is ejected. In some embodiments,the printhead IC is mounted in a printer such that during use thenozzles are less than 100 microns from the paper path.

Preferably, the support structure has a chip mounting area and aconductor mounting area, the die is supported on the chip mounting area,and a plurality of electrical conductors at least partially supported onthe conductor mounting area wherein, the chip mounting area is raisedrelative to the conductor mounting area.

Preferably, the chip mounting area is raised more than 100 micronsrelative to the conductor mounting area. Preferably, the supportstructure has a step between the chip mounting area and the conductormounting area. In some embodiments, the plurality of conductors areincorporated into a flexible printed circuit board (flex PCB) with aline of bond pads along an edge closest the die, the bond pads beingmore than 2 mm from the contacts pads on the die.

Preferably, the support structure is a liquid crystal polymer (LCP)molding.

According to a third aspect, the present invention provides a method forprofiling a bead of encapsulant extending along an edge of a die mountedto a supporting structure, the method comprising the steps of:

depositing a bead of encapsulant onto wire bonds along the edge of thedie;

positioning a profiling surface over the die at a predetermined spacingfrom the die;

moving the profiling surface across the bead before the bead ofencapsulant has cured to reshape the bead profile; and,

curing the bead of encapsulant.

The invention has found that the encapsulant can be effectively shapedby a profiling surface without stripping the encapsulant from the wirebonds. The normally convex-shaped upper surface of the encapsulant beadcan be pushed to one side of the bead with the profiling surface. With alower encapsulant bead, the active surface can be brought into closerproximity with another surface without making contact. For example, thenozzle array on a printhead IC can be 300 microns to 400 microns fromthe paper path. By collapsing or flattening the wire bond arcs beforeapplying and profiling a bead of encapsulant, the nozzle array on theprinthead IC can be less than 100 microns from the paper path.

Preferably, the wire bonds extend in an arc from respective contact padson the die to corresponding conductors on the support structure and themethod further comprises the steps of:

pushing on the wire bonds to plastically deform the wire bonds; and,

releasing the wire bond such that plastic deformation maintains the wirebond in a flatter profile shape.

Preferably, the die has an active surface that has functional elements,the contacts pad being formed at one edge of the active surface, thewire bond has a diameter less than 40 microns and the arc extends morethan 100 microns above the active surface of the die.

Preferably, the wire bond is plastically deformed such that it extendsless than 50 microns above the active surface of the die.

Preferably, the wire bond is pushed by engagement with a blade having arounded edge section for contacting the wire bond.

Preferably, the bead of encapsulant has a profiled surface that is flat,parallel to and spaced less than 100 microns from the active surface.

Preferably, the bead of encapsulant has a profiled surface that is flatand inclined relative to the active surface.

Preferably, the encapsulant being an epoxy material has a viscositygreater than 700 cp when uncured.

Preferably, the encapsulant being an epoxy material that is thixotropicwhen uncured.

Preferably, the method further comprises the steps of:

positioning the profiling surface adjacent and spaced from the activesurface to define a gap; and,

applying the bead of encapsulant onto the contact pads such that oneside of the bead contacts the profiling surface and a portion of thebead extends into the gap and onto the active surface.

Preferably, the active surface has functional elements spaced less than260 microns from the contacts pads of the die. In a further preferredform, the die is an inkjet printhead IC and the functional elements arenozzles through which ink is ejected. In some embodiments, the printheadIC is mounted in a printer such that during use the nozzles are lessthan 100 microns from the paper path.

Preferably, the support structure has a chip mounting area and aconductor mounting area, the die is supported on the chip mounting area,and a plurality of electrical conductors at least partially supported onthe conductor mounting area wherein, the chip mounting area is raisedrelative to the conductor mounting area.

Preferably, the chip mounting area is raised more than 100 micronsrelative to the conductor mounting area. In a particularly preferredform, the support structure has a step between the chip mounting areaand the conductor mounting area.

Preferably, the plurality of conductors are incorporated into a flexibleprinted circuit board (flex PCB) with a line of bond pads along an edgeclosest the die, the bond pads being more than 2 mm from the contactspads on the die.

Preferably, the support structure is a liquid crystal polymer (LCP)molding.

According to a fourth aspect, the present invention provides a method ofapplying encapsulant to a die mounted to a support structure, the methodcomprising the steps of:

providing a die mounted to the support structure, the die having a backsurface in contact with the support structure and an active surfaceopposing the back surface, the active surface having electrical contactpads;

positioning a barrier proximate the electrical contact pads and spacedfrom the active surface to define a gap; and,

depositing a bead of encapsulant onto the electrical contact pads suchthat one side of the bead contacts the barrier and a portion of the beadextends into the gap and onto the active surface.

Placing a barrier over the active surface so that it defines a narrowgap allows the geometry of the encapsulant front (the line of contactbetween the encapsulant and the active surface) can be more closelycontrolled. Any variation in the flowrate of encapsulant from the needletends to cause bulges or valleys in the height of the bead and or thePCB side of the bead. The fluidic resistance generated by the gapbetween the barrier and the active surface means that the amount ofencapsulant that flows into the gap and onto the active surface isalmost constant. The reduced flow variations make the encapsulant frontclosely correspond to the shape of the barrier. Greater control of theencapsulant front allows the functional elements of the active surfaceof the die to be closer to the contact pads.

Preferably, the barrier is a profiling surface and the method furthercomprises the steps of:

moving the profiling surface over the active surface to flatten the beadof encapsulant.

Preferably, the method further comprises the steps of:

prior to depositing the bead of encapsulant, electrically connecting thecontact pads on the die to respective conductors on the supportstructure with wire bonds, the wire bonds each extending in an arc fromthe contact pad to the conductor;

pushing on the wire bonds to collapse the arc and plastically deform thewire bond; and,

releasing the wire bonds such that plastic deformation maintain the wirebonds in a flatter profile shape.

In a further preferred form, the active surface that has functionalelements, the contacts pad being formed at one edge of the activesurface, the wire bond has a diameter less than 40 microns and the arcextends more than 100 microns above the active surface of the die.

Preferably, the wire bond is plastically deformed such that it extendsless than 50 microns above the active surface of the die. In anotherpreferred form, the wire bond is pushed by engagement with a bladehaving a rounded edge section for contacting the wire bond.

Preferably, the bead of encapsulant has a profiled surface that is flat,parallel to and spaced less than 100 microns from the active surface.

Optionally, the bead of encapsulant has a profiled surface that is flatand inclined relative to the active surface.

Preferably, the encapsulant being an epoxy material has a viscositygreater than 700 cp when uncured.

Preferably, the encapsulant is an epoxy material that is thixotropicwhen uncured.

Preferably, the active surface has functional elements spaced less than260 microns from the contacts pads of the die. In a particularlypreferred form, the die is an inkjet printhead IC and the functionalelements are nozzles through which ink is ejected. Preferably, theprinthead IC is mounted in a printer such that during use the nozzlesare less than 100 microns from the paper path.

Preferably, the support structure has a chip mounting area and aconductor mounting area, the die is supported on the chip mounting area,and a plurality of electrical conductors at least partially supported onthe conductor mounting area wherein, the chip mounting area is raisedrelative to the conductor mounting area. In a particularly preferredform, the chip mounting area is raised more than 100 microns relative tothe conductor mounting area. In preferred embodiments, the supportstructure has a step between the chip mounting area and the conductormounting area. In particularly preferred embodiments, the plurality ofconductors are incorporated into a flexible printed circuit board (flexPCB) with a line of bond pads along an edge closest the die, the bondpads being more than 2 mm from the contacts pads on the die.

Preferably, the support structure is a liquid crystal polymer (LCP)molding.

According to a fifth aspect, the present invention provides a method ofapplying encapsulant to wire bonds between a die and conductors on asupporting substrate, the method comprising the steps of:

forming a bead of the encapsulant on a profiling surface;

positioning the profiling surface such that the bead contacts the die;and,

moving the profiling surface relative to the die to cover the wire bondswith the encapsulant.

Wiping the encapsulant over the wire bonds with a profiling surfaceprovides control of the encapsulant front as well as the height of theencapsulant relative to the die. The movement of the profiling surfacerelative to the die can closely controlled to shape the encapsulant to adesired form. Using the example of a printhead die, the encapsulant canbe shaped to present an inclined face rising from the nozzle surface toa high point over the wire bonds. This can be used by the printheadmaintenance facilities to maintain contact pressure on the wipingmechanism. This is illustrated further below with reference to thedrawings. However, it will be appreciated that the encapsulant can beshaped to have ridges, gutters, grooves and so on by using a particularshape of profiling surface and relative movement with the die.

Preferably, the method further comprises the steps of:

dipping the profiling surface into a reservoir of the encapsulantmaterial to form a the bead of encapsulant material on the profilingsurface.

Optionally, the profiling surface is a blade with a straight edge andthe method further comprises the steps of:

orienting the blade such that the straight edge is lowest and dippingthe straight edge into the encapsulant material to form the bead ofencapsulant along the straight edge.

Preferably, the die has an active surface with functional elements and aplurality of contacts pad being formed along one edge for connectionwith the wire bonds such that the wire bonds extend in an arc from thecontacts pads to each of the conductors respectively, the wire bondshaving a diameter less than 40 microns and the arc extends more than 100microns above the active surface of the die.

Preferably, the method further comprises the steps of:

prior to encapsulation, pushing on the wire bonds to collapse the arcand plastically deform the wire bonds; and,

releasing the wire bonds such that plastic deformation maintains thewire bonds in a flatter profile shape.

Preferably, the wire bond is plastically deformed such that it extendsless than 50 microns above the active surface of the die. Preferably,the wire bond is pushed by engagement with a blade having a rounded edgesection for contacting the wire bond.

Preferably, the encapsulant covering the wire bonds has a profiledsurface that is flat, parallel to and spaced less than 100 microns fromthe active surface.

Preferably, the bead of encapsulant having a profiled surface that isflat and inclined relative to the active surface.

Preferably, the encapsulant being an epoxy material has a viscositygreater than 700 cp when uncured.

Preferably, the encapsulant is an epoxy material that is thixotropicwhen uncured. Preferably, the functional elements are spaced less than260 microns from the contacts pads of the die. In a further preferredform, the die is an inkjet printhead IC and the functional elements arenozzles through which ink is ejected. Optionally, the printhead IC ismounted in a printer such that during use the nozzles are less than 100microns from the paper path.

Preferably, the support structure has a chip mounting area and aconductor mounting area, the die is supported on the chip mounting area,and a plurality of electrical conductors at least partially supported onthe conductor mounting area wherein, the chip mounting area is raisedrelative to the conductor mounting area. In a particularly preferredform, the chip mounting area is raised more than 100 microns relative tothe conductor mounting area. In another preferred form, the supportstructure has a step between the chip mounting area and the conductormounting area. In a preferred embodiment, the plurality of conductorsare incorporated into a flexible printed circuit board (flex PCB) with aline of bond pads along an edge closest the die, the bond pads beingmore than 2 mm from the contacts pads on the die. In some embodiments,the support structure is a liquid crystal polymer (LCP) molding.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described by way of exampleonly with reference to the accompanying drawings, in which:

FIG. 1 is a schematic representation of a common prior art technique forapplying a bead of encapsulant to wire bonds;

FIG. 2 is a schematic representation of a die mounted to a supportingstructure with a chip mounting area raised relative to the flex PCBmounting area;

FIGS. 3A, 3B and 3C are schematic representations of the encapsulantbead being profiled into a desired shape using a moveable blade;

FIGS. 4A to 4D are schematic representations of wire bonds beingprofiled by plastic deformation;

FIGS. 5A and 5B show the encapsulant bead height reductions forplastically deformed wire bonds;

FIGS. 6A to 6C show the encapsulant bead being applied to the wire bondsusing the profiling blade; and,

FIGS. 7A and 7B show the profiling blade being used to control theencapsulant bead front on the surface of the die.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

FIG. 1 shows a common technique used for applying a bead encapsulant towire bonds. A die 4 is mounted to a supporting structure 6 adjacent theedge of a flex PCB 8 (flexible printed circuit board). The die 4 has aline of contact pads 10 along one edge and the flex PCB 8 hascorresponding bond pads 12. Wire bonds 16 extend from the bond pads 10to the bonds pads 12. Power and data is transmitted to the die 4 viaconductive traces 14 in the flex PCB 8. This is a simplifiedrepresentation of the dies mounted within many electronic devices. Theprinthead IC dies mounted to the LCP (liquid crystal polymer) molding toreceive print data from an adjacent flex PCB, as described in U.S. Ser.No. ______ (Our Docket RRC001US) incorporated herein by cross reference,is one example of this type of die mounting arrangement. The ordinaryworker will appreciate that the die may also be mounted directly to ahard PCB with traces formed thereon.

The wire bonds 16 are covered in a bead on encapsulant 2 to protect andreinforce the bonds. The encapsulant 2 is dispensed from a dischargeneedle 18 directly onto the wire bonds 16. Often the encapsulant bead 2is three separate beads—two beads of so-called ‘dam’ encapsulant 20, andone bead of ‘fill’ encapsulant 22. The dam encapsulant 20 has a higherviscosity than the fill encapsulant 22, and serves to form a channel tohold the fill encapsulant bead. The height H of the bead 2 above the die4 is usually about 500-600 microns. In most electronic devices, thisdoes not pose a problem. However, if the die has an active surface thatneeds to operate in close proximity to another surface, this bead can bean obstruction.

Elevating the Die Relative to the Flex PCB

FIG. 2 shows a stepped support structure 6 that has raised the chipmounting area 26 relative to the PCB mounting area 24 (or at least thearea mounting the PCB bonds pads 12). With the die 4 on a raised chipmounting area 26, the arc of the wire bonds 16 are lower relative toactive surface 28 of the die 4. In fact, the end of the wire bond 16attached to the contact pad 10 can be the apex of the arc (bearing inmind that the wire bond arc is intended to accommodate some relativemovement of the die and PCB). When the wire bonds 16 are covered withencapsulant 2, the bead has a reduced height H above the active surface28 of the die 4. If the bead of encapsulant 2 uses two beads of damencapsulant 24 and a fill encapsulant 22, the positions, volumes andviscosities of the beads need to take the step into account. Beadheights less than 100 microns are easily achievable, and with additionalmeasures, such as wire arc collapsing and bead profiling (discussedbelow), bead height of less than 50 microns are possible.

With the die 4 raised above the flex PCB 8 by 410 microns, the height ofthe wire bonds 16 above the die is about 34 microns. With the die raised610 microns above the flex PCB, the wire bond height is around 20microns. Raising the die even further has shown little or no furtherreduction in wire bond height with a step of 710 microns having a wirebond height of around 20 microns.

Shaping the Encapsulant Bead with a Profiling Blade

FIGS. 3A to 3C show the encapsulant 2 being profiled with a profilingblade 30. The support structure 6 is again stepped to reduce the heightof the wire bonds 16 above the die 4. Before the epoxy encapsulant 2 hascured, the profiling blade 30 moves across the die 4 and wire bonds in apredetermined path. As shown in FIG. 3B, the blade 30 displaces the topof the bead 30 to its flex PCB side to form a flat top surface 32 thatis at a significantly reduced height H above the die 4.

The encapsulant bead 2 may be a plurality of separate beads as shown inFIGS. 1 and 2, or a single bead of one material. However, for closedimensional control of the profiled encapsulant, the encapsulantmaterials used should be thixotropic—that is, once deposited from thedischarge needle, or profiled by the blade 30, the material should notflow under its own weight, but rather hold its form until it cures. Thisrequires the epoxy to have an uncured viscosity greater than about 700cp. A suitable encapsulant is DYMAX 9001-E-v3.1 Chip Encapsulantproduced by Dymax Corporation with a viscosity of approximately 800 cpwhen uncured. The blade 30 may be ceramic (glass) or metal andpreferably about 200 microns thick.

It will be appreciated that the relative movement of the blade 30 andthe die 4 can be precisely controlled. This allows the height H to bedetermined by the tolerance of the wire bonding process. As long as H isgreater than the nominal height of the wire bond arc above the die, plusthe maximum tolerance, the encapsulant 2 will cover and protect the wirebonds 16. With this technique, the height H can be easily reduced from500-600 microns to less than 300 microns. If the heights of the wirebond arcs are also reduced, the height H of the encapsulant bead can beless than 100 microns. The Applicant uses this technique to profileencapsulant on printhead dies down to a height of 50 microns at itslowest point. As shown in FIG. 3C, the lowest point is at theencapsulant front and the blade 30 forms an inclined face 32 in the topof the bead 2. The inclined face is utilized by the printheadmaintenance system when cleaning the paper dust and dried ink from thenozzle face. This illustrates the technique's ability to not just reducethe height of the encapsulant bead, but to form a surface that canperform functions other than just encapsulate the wire bonds. The edgeprofile of the blade and the path of the blade relative to the die canbe configured to form a surface that has a multitude of shapes for avariety of purposes.

Plastic Deformation of the Wire Bond Arcs

FIGS. 4A to 4C show another technique for lowering the profile of wirebonds. FIG. 4A shows the die 4 connected to the flex PCB 8 via the wirebonds 16. While the stepped support structure 6 has lowered the heightof the wire bond arcs compared to a flat supporting structure, the wirebonds still have a natural tendency to bow upwards rather than downwardstowards the corner of the step. The wires 16 are typically about 32microns in diameter and have a pull force of about 3 to 5 grams force.The pull force is the tensile load necessary to break the connection tothe contact pad 10 or the bond pad 12. Given the fragility of thesestructures (one of the reasons encapsulant is applied), conventionalwisdom is to avoid any contact between the wire bond arcs and othersolid surfaces.

As shown in FIG. 4B, the arc of the wire bonds 16 can be collapsed by awire pusher 34. The wire pusher 34 displaces the wire bond 16 enough toelastically and plastically deform the arc. The Applicants have shownthat contact with the wire pusher 34 can cause localized work hardeningin the wire, but as long as the pushing force is not excessive, it doesnot break. The end of the wire pusher 34 is rounded to avoid stressconcentration points. The wire pusher may be a stylus for engagingsingle wire bonds or a blade that pushes on multiple wire bondssimultaneously.

Referring now to FIG. 4C, the wire pusher 34 is retracted and the wiresprings back toward its original shape to relieve the elasticdeformation. However, the plastic deformation remains and the wire bondheight above the die 4 is much reduced. Testing has shown that aninitial wire bond loop height of 200 microns can be reduced to about 35microns using this technique. Tests have also shown that the pullstrength of the plastically deformed wires remains at about 3 to 5 gramsforce.

The collapse of the wire bonds is uncontrolled and leaves the wire bondssomewhat randomly deformed. However, pushing the wire bonds closer tothe die provides more uniformly shaped collapsed wire bonds. TheApplicant's work has shown that engaging the wires about 200 to 300microns for the die provides the best results.

As shown in FIG. 4D, the die 4 and the flex PCB 8 are mounted to a flatsupport structure 6. As discussed above, this means the original loopheight of the wire bond arc is much higher—approximately 400 micronsabove the die 4. Consequently, the wire has more plastic deformationwhen the loop is collapsed by the wire pusher. Even so, the Applicantsresults show that the residual loop height after pushing is about 20-50microns.

FIGS. 5A and 5B show the collapsed wire bonds 16 covered with anencapsulant bead 2. Even without bead profiling prior to curing, theheight H of the bead above the die is much less than the bead necessaryto encapsulate the original undeformed wire loops.

Applying Encapsulant with Profiling Blade

FIGS. 6A, 6B and 6C show the application of the encapsulant bead usingthe profiling blade 30 instead of a discharge needle (see FIGS. 1 and2). As previously discussed, the flowrate of encapsulant from thedischarge needle can vary and this gives rise to large variations on theposition of the encapsulant front on the active surface of the die 4.Consequently, any functional elements in the active surface of the dieneed to be sufficiently spaced from the contacts pads 10 to allow forthe meandering encapsulant front.

Applying the encapsulant with the profiling blade avoids the problemscaused by the flowrate fluctuations from the discharge needle. As shownin FIG. 6A, the bead of encapsulant 40 can be formed on the profilingblade 30 by simply dipping it into a reservoir of uncured encapsulantepoxy. Of course, the bead 40 may also be formed by any other convenientmethod, such as running the discharge needle along one end of the blade30.

FIG. 6B show the blade 30 having been lowered to touch the bead 40 ontothe die 4. When the encapsulant material touches the die surface, itwets and wicks along the surface while remaining pinned to the edge ofthe blade. The blade 30 is held at a predetermined height above the die4 and moved over the bead 2 to flatten and lower its profile. Theencapsulant displaced from the top of the bead 2 by the blade 30,spreads over the PCB side of the bead 2. It is not relevant if theencapsulant spreads further over the PCB than necessary. As long as thewire bonds 16 and the bonds pads 12 are covered, any additionalencapsulant on the PCB 8 surface is not detrimental.

In FIG. 6C, the wire bond 16 height has been reduced by collapsing thearc in accordance with the techniques discussed above. As previouslydiscussed, the bead 2 deposited by the discharge needle need not be asbig to cover the wire bond 16 once it has been collapsed. Furthermore,the blade 30 can be brought closer to the die 4 without contacting wirebonds 16 when profiling the encapsulant 2. Hence the bead profile inFIG. 6C is substantially lower than that of FIG. 6B.

Encapsulant Front Control

When the encapsulant material is dispensed from the discharge needle,minor variations in the flowrate can cause the bead to bulge at pointsof higher flow. Consequently, the side of the bead that contacts theactive surface of the die is not straight, but has significantperturbations. These perturbations have to be accommodated between thecontact pads and any functional elements on the active surface. Thespacing between the contacts pads and the functional elements consumesvaluable ‘chip real estate’. The Applicant has previously developedprinthead dies with a spacing of 260 microns between the contact padsand the first row of nozzles. Better control of the encapsulant frontreduces the space between the contacts and operational elements, and sothe overall dimensions of the die. Hence the design can be more compactand more chips fabricated from the original wafer disc.

As shown in FIGS. 7A and 7B, the profiling blade 30 is used to controlthe front 36 of the bead of encapsulant 2. The blade 30 is positionedover the die 4 to define a gap 42 between its lower edge and the activesurface 28. As the discharge needle 18 dispenses the encapsulantmaterial 44, it flows onto the active surface, one side of the blade anda fillet of the material extends through the gap 42. Because of the flowrestriction created by the gap, flow variations have a reduced effect onthe dimensions of the fillet that flows through the gap. Therefore theencapsulant front 36 closely corresponds to the line of the lower edgeof the blade 30.

As shown in FIG. 7B, the profiling blade 30 is already in position toprofile the encapsulant bead 2 once it has been dispensed from thedischarge needle. The blade 30 simply moves over the die 4 in adirection away from the nozzles 38. This keeps the encapsulant front 36in place and flattens the profile of the encapsulant bead 2 over thewire bonds 16.

The invention has been described herein by way of example only. Theordinary will readily recognize many variations and modifications whichdo not depart from the spirit and scope of the broad inventive concept.

1. A method of profiling a wire bond between a contact pad on a die, anda conductor on a supporting structure, the method comprising the stepsof: electrically connecting the contact pad on the die to the conductoron the supporting structure with a wire bond, the wire bond extending inan arc from the contact pad to the conductor; pushing on the wire bondto collapse the arc and plastically deforming the wire bond; and,releasing the wire bond such that plastic deformation maintains the wirebond in a flatter profile shape.
 2. A method according to claim 1wherein the die has an active surface that has functional elements, thecontacts pad being formed at one edge of the active surface, the wirebond has a diameter less than 40 microns and the arc extends more than100 microns above the active surface of the die.
 3. A method accordingto claim 2 wherein the wire bond is plastically deformed such that itextends less than 50 microns above the active surface of the die.
 4. Amethod according to claim 1 wherein the wire bond is pushed byengagement with a blade having a rounded edge section for contacting thewire bond.
 5. A method according to claim 1 further comprising the stepsof: applying a bead of encapsulant over the wire bond; and, moving aprofiling surface over the active surface to flatten the bead ofencapsulant.
 6. A method according to claim 5 wherein the bead ofencapsulant having a profiled surface that is flat, parallel to andspaced less than 100 microns from the active surface.
 7. Amicroprocessor device according to claim 5 wherein the bead ofencapsulant having a profiled surface that is flat and inclined relativeto the active surface.
 8. A method according to claim 5 wherein theencapsulant being an epoxy material has a viscosity greater than 700 cpwhen uncured.
 9. A method according to claim 5 wherein the encapsulantbeing an epoxy material that is thixotropic when uncured.
 10. A methodaccording to claim 5 further comprising the steps of: positioning theprofiling surface adjacent and spaced from the active surface to definea gap; and, applying the bead of encapsulant onto the contact pads suchthat one side of the bead contacts the profiling surface and a portionof the bead extends into the gap and onto the active surface.
 11. Amethod according to claim 10 wherein the active surface has functionalelements spaced less than 260 microns from the contacts pads of the die.12. A method according to claim 10 wherein the die is an inkjetprinthead IC and the functional elements are nozzles through which inkis ejected.
 13. A method according to claim 12 wherein the printhead ICis mounted in a printer such that during use the nozzles are less than100 microns from the paper path.
 14. A method according to claim 1wherein the support structure has a chip mounting area and a conductormounting area, the die is supported on the chip mounting area, and aplurality of electrical conductors at least partially supported on theconductor mounting area wherein, the chip mounting area is raisedrelative to the conductor mounting area.
 15. A method according to claim14 wherein the chip mounting area is raised more than 100 micronsrelative to the conductor mounting area.
 16. A method according to claim15 wherein the support structure has a step between the chip mountingarea and the conductor mounting area.
 17. A method according to claim 15wherein the plurality of conductors are incorporated into a flexibleprinted circuit board (flex PCB) with a line of bond pads along an edgeclosest the die, the bond pads being more than 2 mm from the contactspads on the die.
 18. A method according to claim 17 wherein the supportstructure is a liquid crystal polymer (LCP) molding.